Ultrasonic diagnostic transducer array with elevation focus

ABSTRACT

An ultrasonic diagnostic transducer array is provided for providing electronic focusing in the longitudinal plane and elevational focusing. Elements of the array are subdiced in the elevational direction to provide subelements with aspect ratios varying in proportion to their distance to the central longitudinal axis of the array. Such variation affords varying electro-mechanical coupling coefficients to the subelements such that the intensity of the transmitted energy is centered about the central longitudinal axis. In a second embodiment elements exhibit extensions in the elevational direction which vary in proportion to their displacement from the longitudinal center of the array. The extended elements are acoustically separated into subelements in the elevational direction to provide elevational focusing or spatial compounding of the transmitted acoustic energy.

This invention relates improvements in transducer arrays used forultrasonic diagnostic imaging, and in particular to ultrasonictransducer arrays which are focused in the elevation direction.

The use of transducer arrays, in which a group of individual elementsare electronically actuated and sampled to steer and focus a beam ofultrasonic energy, is well known. The elements of an array may compriserings which are concentrically arranged to form an annular array. Thepresent invention relates to linear arrays in which the elements arephysically arranged in a straight line, conventionally used for lineararray or phased array imaging. The linear array may also be curved inthe dimension of the imaging plane so that the beams are concurrentlymechanically spread in a fan shaped imaging plane. These linear arraysare ideal for scanning and forming images in a planar region in front ofthe array.

The longitudinal disposition of the array elements permits the beam ofthe array to be electronically focused into a narrow beam in the planeof the image. The single row of elements of the array does not enableelectronic focusing in the transverse, thickness dimension of the plane,which is often desirable in order to obtain high resolution of a thinimage "slice". The conventional technique for restricting the beam to athin image plane is to mechanically focus the beam in this transverse,or elevational, dimension, either by contouring the elements in thisdimension or lensing each element. More recently it has been shown thatelevational focusing can be achieved by controlling the piezoelectricproperties of the elements in this dimension. In this technique, knownas shaded polarization, intense, gradated electric fields are uniformlyapplied to each element to taper the polarization of the piezoelectricelements so that they are most strongly polarized in the center andpolarized to a lesser degree toward each end of the element in theelevational direction. The technique shapes the acoustic transmissivityof each element to be greater along the longitudinal center line of thearray and lesser toward each elevational side. A significantdisadvantage of the technique is the difficulty of precisely controllingthe magnitude and gradient of the polarization shading.

It is also known to accomplish elevational focusing electronically,applying the same principle that is used to focus the beamlongitudinally. Second, third, and additional rows of elements may bearranged along side and parallel to the first longitudinal row ofelements. This forms individual rows of elements in the elevationaldirection and the timed actuation and sampling of these elements enablesthe electronic focusing of the beam in the elevational dimension. But itmay be seen that the electronic approach greatly increases thecomplexity of the ultrasound system. The number of elements of the arraywill triple or better: a 128 element array becomes a two dimensionalarray of 384 or more elements. There is a corresponding increase in thenumber of transmitters and receivers required to operate the twodimensional array, which will greatly increase the cost of the system.

Accordingly, it is desirable to provide a transducer array whichexhibits improved performance over conventional mechanical elevationalfocusing techniques, while avoiding the greatly increased cost andcomplexity of electronic elevational focusing.

In accordance with the principles of the present invention, a transducerarray is provided for focusing of the ultrasonic beam in the elevationaldirection. In a first embodiment, the transducer array comprises acomposite structure of elements of piezoelectric material and a bondingmatrix of non piezoelectric material. The electro-mechanical couplingcoefficients of the elements of piezoelectric material are controlled bycontrolling their respective aspect ratios in a manner such thatelements along the longitudinal centerline of the array exhibit greaterelectromechanical coupling than do elements toward the longitudinaledges of the array. Control of the aspect ratios of the piezoelectricelements in the composite structure thereby affords a focusing of theacoustic beam in the elevational direction. In a second embodiment, theacoustic aperture of the transducer array is expanded longitudinally asthe array is focused at increasing depths. As the aperture expandslongitudinally by the addition of an increased number of activeelements, the aperture is correspondingly expanded in the elevationdirection. In a preferred embodiment, the expansion causes the acousticaperture to be wider at the longitudinal ends of the array than in thecenter, and with the most longitudinal elements to be formed oflaterally separated but electronically common subelements.

IN THE DRAWINGS

FIG. 1 is a top plan view of a linear array of piezoelectric transducerelements;

FIG. 2 is a top plan view of a linear array of piezoelectric transducerelements in which each element has been subdiced;

FIG. 2A is an enlarged edge view of the subdiced array of FIG. 2;

FIG. 3 is a top plan view of a transducer array which is focused in theelevation direction in accordance with the principles of the presentinvention;

FIG. 4 is a side view of the transducer array of FIG. 3;

FIG. 5 is a top plan view of a second embodiment of a transducer arraywhich is focused in the elevation direction in accordance with theprinciples of the present invention;

FIG. 5A is an edge view of one technique for fabricating the array ofFIG. 5, in which varying thicknesses are employed;

FIG. 6 is a top plan view of a modification of the transducer layoutconfiguration of the array of FIG. 5;

FIG. 7 is a perspective view of the focusing effect of the transducerarray of FIG. 6; and FIG. 8 illustrates the profiles of the transmittedacoustic beams in the elevation dimension of the transducer arrays ofFIGS. 5 and 6.

Referring first to FIG. 1, a conventional linear array 10 ofpiezoelectric transducer elements is shown. The array 10 is comprised ofa plurality of individual transducer elements labeled e₁, e₂, e₃, and soforth in the drawing. The drawing also indicates two orientations of thearray, the longitudinal direction shown by arrow L, and the elevationaldirection shown by arrow E. The plane in which the array operatesprojects outward from the center of the face of the array, in parallelwith the longitudinal arrow L. As is well known, the transmitted beamemitted in the operating plane can be focused in the longitudinaldirection by actuating elements in a group of elements at nearlysimultaneous but slightly different times. Through such timed ultrasonictransmission the emitted beam can be focused to a desired point orsteered in a desired direction.

Such focusing or steering is not possible in the elevational direction,however, as there is only a single element in the elevational direction.Electronic steering and focusing in the elevational direction requiresadditional rows of elements, which provides a two dimensional array andincreases the complexity of the driving and receiving electronics by asignificant factor. For a single row, the only practical elevationalfocusing to be obtained is through mechanical effects such as curving orlensing the array in the elevational direction as described in U.S. Pat.No. 3,936,791, or through the technique of shaded polarization referredto previously.

FIG. 2 shows a plan view of the transmitting surface of a transducerarray 12 similar to the array of FIG. 1, but with each element subdiced.A side view of the array of FIG. 2 is provided in FIG. 2A, which furthershows the reference potential electrode 14 on the emitting surfaces ofthe elements and the actuating electrodes 15, 17, 19 on the opposingsurfaces of each pair of subdiced elements. In array 12 each elementcorresponding to e₁, e₂, etc. of FIG. 1 has been subdiced into twosubelements, such as e_(1A) and e_(1B) ; e_(2A) and e_(2B) ; and soforth. From a comparison of FIG. 2 to FIG. 1, it is seen that thetransmitting surface area of each subdiced element is half that of eachoriginal full element, which changes the aspect ratio of each discreteelement. The aspect ratio change of significance to the performance ofeach element is the ratio of the transmitting surface width, indicatedby W in FIG. 2A, to the thickness of the element between the electrodes,indicated by the arrow marked T in FIG. 2A. It is this ratio whichdetermines the electro-mechanical coupling coefficient of the element,which is the magnitude of acoustic energy that will result from a givenquantum of actuating energy. In the array 12, it is desired to createmechanical action in the direction of the thickness dimension T in orderto transmit an acoustic wave outward from the transmitting surface ofthe elements. As the aspect ratio is varied, the transfer of electricalenergy into mechanical acoustic energy is improved, as measured by anenhancement of the electro-mechanical coupling coefficient of theelement. With other factors being equal, it is to be expected that thesubdiced elements e_(1A) and e_(1B) would more efficiently convertelectrical energy into acoustic energy than would the correspondingelement e₁ of FIG. 1. The array 12 is operated by actuating the subdicedpairs together, which is the reason that the energizing electrodes 15,17, 19 bridge pairs of subdiced elements.

In accordance with the principles of the present invention, theelectro-mechanical coupling coefficient of elements of a transducerarray of composite material is varied in the elevational direction inorder to achieve a transmitted energy profile which is focused in theelevation direction. A composite transducer is one in whichpiezoelectric material is suspended in a non piezoelectric matrix. Atransducer array 16 which illustrates the application of theseprinciples is shown in FIGS. 3 and 4. In the plan view of thetransmitting surface of the array 16 of FIG. 3, the array 16 iscomprised of a number of elements e₁, e₂, and so forth. Each element issubdiced in the elevational direction into four subelements. Element e₁is subdiced in four subelements e_(1A), e_(1B), e_(1C), and e_(1D). Asin the case of array 12, this subdicing improves the electro-mechanicalcoupling coefficient of the element e₁ in comparison with operation ofthe element as a single, unitary structure. The manner in which theaspect ratio of a transducer element is varied to achieve a desiredchange in the electromechanical coupling coefficient is materialdependent. This means that a particular aspect ratio change of anelement of one material may affect the coupling coefficient differentlythan that of another material, when the an element of the other materialis subjected to the same aspect ratio change.

Advantage is taken of the elevational subdicing by bidimensionallysubdicing the elements longitudinally, and at intervals such that thewidth of the surface area of the twice subdiced elements in theelevational direction decreases in relation to distance from the centerof the element in that direction. In the figuresthe central subelementhas dimensions indicated as 5-2. In this embodiment the centralsubelement is five units wide in the elevational direction and two unitsacross its longitudinal direction The subelements on either side of thecentral subelement have dimensions indicated as 4-2, which in thisembodiment designates a width of four units in the elevational directionand two units across the longitudinal direction. The subelementsextending outward from the center have dimensions which continue todecline in this manner: 3-2, 2-2, and 1-2. It is seen that the aspectratio of the subelements changes from the central subelement 5-2 to theedge subelements 1-2. The change is symmetrical about the centralsubelement.

As FIG. 4 shows, all of these subelements have the same thickness T.Thus, the aspect ratios change from the central subelement outward. Thechanging aspect ratios affect a decline in the electromechanicalcoupling coefficients of the subelements from the central subelementthrough those at the edges in the elevational direction. FIG. 4 alsoshows that the actuating electrodes 21, 23, 26, 24, and 22 are allconnected in common by a wire 28 soldered to each subelement. Anelectrical pulse applied to wire 28 will cause the central subelement(5-2) to transmit a greater intensity of acoustic energy that any othersubelement, and the quantum of acoustic energy emitted by thesubelements declines as one proceeds to the edges (1-1). Thus, while allsubelements are actuated to transmit acoustic energy simultaneously, theprofile of the transmitted energy will be concentrated at theelevational center of the element as indicated by large arrow C incomparison with smaller arrows S at the sides of the element, therebyeffecting a focusing of the transmitted energy in the elevationaldirection. The focusing is achieved by the above described selection ofthe aspect ratios of the subelements.

Each actuating electrode 21-26 extends across all four of subdicedelements e1A-e1D in the longitudinal direction. All of the subelementsof element e1 are thus actuated in unison. The subdicing in theelevational direction effects an efficient transfer of electricalactuating energy into acoustic energy, and the varying aspect ratios ofthe subelements from the center outward focuses the energy in theelevational direction.

In a constructed embodiment, the voids 20 between the subelements can beair filled, or filled with a non piezoelectric bonding compound such asepoxy to retain the subelements in the matrix. FIG. 4 also shows thearray backed by a backing of filler material 18 as a damping material.

FIG. 5 illustrates a second embodiment of the present invention whichadds several further concepts to achieve elevational focusing. FIG. 4shows an array 30 comprised of subarrays 32-40. A central subarray 32 isflanked on either side in the longitudinal direction by pairs ofsubarrays 34a-34b and 36a-36b. Like the embodiment of FIGS. 3 and 4, thecentral subarray 32 has a greater aspect ratio than any of the flankingsubarrays, with the flanking subarrays extending further out in theelevational direction. In addition, the flanking subarray pairs areseparated from each other. The subarrays 34a-34b and 36a-36b are flankedin the longitudinal direction by two additional pairs of subarrays,38a-38b and 40a-40b. The subarrays in each of these pairs are separatedby an even greater distance in the elevational direction than are theinner pairs 34a-34b and 36a-36b.

The array 30 is operated by actuating different subarray combinationsdepending upon the depth of field at which the acoustic beam is to befocused. The aperture of the array is expanded as acoustic waves aretransmitted to increasing depths. In the near field the subarray 32 isoperated alone, without use of any of the other subarrays. In anintermediate depth of field the flanking subarrays 34a-34b and 36a-36bare used in concert with the central subarray. The addition of theseflanking subarrays expands the active aperture of the array and are usedtogether with the central array to focus and steer the transmitted beamin the longitudinal direction through the timed actuation of theindividual elements in the subarrays. Elevational focusing is achievedin two ways. First, the greater aspect ratio and electro-mechanicalcoupling coefficient of the elements of the central subarray 32 causethe central subarray to emit greater acoustic energy than the flankingsubarrays for the same level of actuating energy. Second, the separationof corresponding subarrays in the flanking pairs, in combination with aproper excitation time delay, contribute components of acoustic energywhich focus the acoustic beam in the elevational direction toward thelongitudinal center of the array.

When far field transmission is required all of the subarrays come intoplay. The aperture of the array 30 is thereby expanded to its maximum inthe longitudinal direction. The outermost subarrays 38a-38b and 40a-40bcontribute the same elevational focusing effect as the flankingsubarrays 34a-34b and 36a-36b, but to an even greater degree by virtueof their even greater outward positions in the elevational direction.Thus, a beam which is focused in both the longitudinal and elevationaldirections can be transmitted to the maximum operating depth of field ofthe array 30.

In construction of the array 30, the actuating electrodes of verticallyopposing elements of the paired subarrays are connected electrically incommon. While the elements of the array may be operated separately atphased actuation times in the longitudinal direction, the correspondingelements of matched subarrays can be actuated in unison to achieve thedesired focusing effect in the elevational direction.

A variation of the array 30 of FIG. 5 is shown in the embodiment ofFIGS. 6 and 7. In FIG. 6, the outline 42 represents a bar ofpiezoelectric material which has been diced as indicated by the lineswithin the outline, but with only the shaded subelements being connectedto actuating electrodes and operable. A central subarray e_(c) extendsfrom element e₇ through element e₁₆. The central subarray ec is operatedwhen the array transmits acoustic energy in the near field. As the depthof field increases, the longitudinally flanking subelements are added,beginning with subelements e_(6A) and e_(6B), and e_(17A) and e_(17B).With increasing depth of field the other subelements are added,expanding the aperture until the full aperture of e_(1A),e_(1B) throughe_(22A),e_(22B) is employed at the greatest depth. Like the previousembodiment, the central subarray elements have a greater aspect ratiothan do the paired subelements to provide greater intensity at thecenter of the array. The outward angular inclination of the pairs ofseparated subelements gives a gradual increase in the elevationalfocusing effect of the subelement pairs as the aperture is expanded withincreasing depth of field.

These effects are illustrated in the far field in the perspective viewof the array 42 and its transmit plane in FIG. 7. The transmit plane 60is normal to the transmitting face of the array and aligned with itscentral longitudinal axis. A center line CL of the plane 60 extends fromthe center of the central subarray e_(c). In the drawing figure thearray 42 is focused at a point F and all of the elements of the arrayare utilized. To focus the transmitted beam longitudinally, theoutermost elements are actuated first, and the sequence of actuationproceeds inwardly until the elements in the center of the array areactuated last. Elevational focusing, which focuses the transmitted beamtoward the plane 60, is achieved by the separation of the upwardlyextending subelements 52 and their opposition by the downwardlyextending subelements 54, and the corresponding separation ofsubelements 56 and 58. The dashed lines from the corners of the arrayindicate the effect of this elevational focusing.

FIG. 8 illustrates exemplary acoustic beam profiles for the embodimentsof FIGS. 5-7. In this drawing the transducer array is viewed edge on,with the transmitted beams extending to the right. In the near field thecentral elements e_(c) are used alone to focus a beam P₁ at a near fieldfocal point F₁. The elements e_(c) in this drawing correspond to thecentral subarray 32 in FIG. 5 or the central elements e_(c) in FIG. 6.In the far field the central elements e_(c) are used together with theoutwardly extending paired elements e_(nA) and e_(nB) to produce a beamP₂ which is focused at a point F₂. The array has good elevational focuscharacteristics in both the near and the far field.

In the embodiment of FIGS. 6 and 7, like that of FIG. 5, verticallyaligned elements (elements which align in the elevational direction) areelectrically connected together and operated in unison. If desired,however, the actuating electrodes of the vertically aligned elements canbe electrically separated and the elements actuated independently ofeach other. This would provide the opportunity for deriving additionaloperational benefits from the array. Referring to FIG. 7 for instance,it would then be possible to actuate the upward extending subelements52, 56 at a slightly different time than the corresponding downwardextending subelements 54, 58. This would cause acoustic energy from therespective extending lines of subelements to arrive at the focal pointand return from a target at the focal point at slightly different timesand phase relationships. Such timing and phase differences wouldconstitute a spatial compounding of the acoustic beam that would disruptthe usual interaction of acoustic waves that leads to development of thefamiliar speckle pattern in ultrasonic images. This mode of operation ofthe array would enable formation of ultrasonic images with reducedspeckle content in comparison with simultaneous operation of theopposing subelements.

A second approach to reducing speckle content is illustrated by FIG. 5A,which is an edge-on view of the array of FIG.5. In FIG. 5A the subarrays32, 34 and 36, and 38 and 40 exhibit different thicknesses, providingthe respective subarrays with different frequency responses. The centralsubarray has the highest frequency response, the subarrays on eitherside have a lower frequency response, and the outermost subarrays havethe lowest frequency response. As the outer subarrays are added to theaperture at increasing depths, the subarrays with the lower frequenciesbecome active, transmitting and receiving ultrasonic signals. When theresulting electrical signals from all active subarrays are combined toform a beam, the differing received signal frequencies are compounded,providing reduced speckle content in the resultant beam.

What is claimed is:
 1. A linear array of ultrasonic transducer elementswhich transmits acoustic beams in a plane extending from a centrallongitudinal axis of said array, including a plurality of uniformelements arrayed across the full aperture in the longitudinal directioncomprising subelements arrayed in the elevational direction andexhibiting varying aspect ratios such that the electro-mechanicalcoupling coefficients of the subelements vary as a function of theirdistance from said central longitudinal axis whereby an acoustic beamwhich varies in intensity in the elevational direction may be producedby said array.
 2. The array of ultrasonic transducer elements of claim1, wherein said array comprises a composite array in which saidsubelements comprise piezoelectric ceramic material and the intersticesbetween said subelements are filled with a non piezoelectric material.3. The array of ultrasonic transducer elements of claim 2, wherein saidnon piezoelectric material comprises an epoxy material.
 4. The array ofultrasonic transducer elements of claim 1, wherein all of thesubelements of an element of said array have first and second electrodeslocated on opposite surfaces of said subelements, said first electrodesare electrically coupled in common and said second electrodes areelectrically coupled in common.
 5. The array of ultrasonic transducerelements of claim 4, wherein said first electrodes are located on thetransmitting surfaces of said subelements and are coupled to a referencepotential, and said second electrodes are located on the oppositesurfaces of said subelements and are coupled to a switched actuatingpotential.
 6. An array of ultrasonic transducer elements which transmitsacoustic beams in a plane extending from a central longitudinal axis ofsaid array, including a plurality of elements of uniformelectro-mechanical coupling coefficients in the longitudinal directionarrrayed across the full aperture in the longitudinal directioncomprising subelements arrayed in the elevational direction, saidsubelements exhibiting varying aspect ratios such that theelectro-mechanical coupling coefficients of the subelements vary as afunction of their distance from said central longitudinal axis, wherebyan acoustic beam which varies in intensity in the elevational directionmay be produced by said array,wherein said aspect ratios of saidsubelements vary such that subelements closer to said centrallongitudinal axis exhibit a greater electro-mechanical couplingcoefficient than subelements which are further removed from said centrallongitudinal axis.
 7. The array of ultrasonic transducer elements ofclaim 6, wherein all of the subelements which are arrayed in a givenline in the elevational direction exhibit a common thickness, a commonwidth in said longitudinal direction, and varying lengths in theelevational direction.
 8. The array of ultrasonic transducer elements ofclaim 7, wherein each element of said array comprises a plurality ofsaid lines of subelements which are arrayed in the elevationaldirection.
 9. An array of ultrasonic transducer elements which transmitsacoustic beams in a plane extending from a central longitudinal axis ofsaid array, including a plurality of elements which are identical in thelongitudinal direction and arrayed in the longitudinal directioncomprising subelements arrayed in the elevational direction andexhibiting varying aspect ratios such that the electro-mechanicalcoupling coefficients of the subelements vary as a function of theirdistance from said central longitudinal axis, whereby an acoustic beamwhich varies in intensity in the elevational direction may be producedby said array,wherein the electro-mechanical coupling coefficients of aline of subelements arrayed in the elevational direction vary as afunction of their distance from said central longitudinal axis withsubelements more closely located to said central longitudinal axisexhibiting a greater electro-mechanical coupling coefficient thansubelements which are further removed from said central longitudinalaxis, whereby an acoustic beam which exhibits a greater intensity at theelevational center of said array than at the elevational edges may beproduced by said array.
 10. A linear array of ultrasonic transducerelements arrayed in the longitudinal direction of said array whichtransmits acoustic beams in a plane extending from a centrallongitudinal axis of said array, comprising:a first group of elementslocated about the longitudinal center of said array and operable for thetransmission of acoustic energy in the near and far fields; and secondand third groups of elements located entirely on opposite sides of saidfirst group of elements in the longitudinal direction, with elements ofsaid second and third groups extending a greater distance in theelevational direction from said central longitudinal axis than saidfirst group of elements, wherein said second and third groups ofelements are operable in concert with said first group of elements toprovide elevational focusing in the far field.
 11. The array ofultrasonic transducer elements of claim 10, wherein each of said secondand third groups of elements comprises first and second groups ofsubelements which oppose each other in the elevational direction and areacoustically separated from each other.
 12. The array of ultrasonictransducer elements of claim 11, wherein opposing subelements of saidfirst and second groups of subelements are actuated in common.
 13. Thearray of ultrasonic transducer elements of claim 12, wherein saidelements and subelements are separately actuatable from each other inthe longitudinal direction for steering and focusing of said beam in thelongitudinal direction of said plane.
 14. The array of ultrasonictransducer elements of claim 11, wherein the element of said first groupexhibit a greater length in the elevational direction than do saidsubelements of said first and second groups of subelements.
 15. Thearray of ultrasonic transducer elements of claim 11, wherein said firstgroup of subelements of each of said second and third groups of elementsis located about a second longitudinal axis of said array which isoffset on one side of said central longitudinal axis in the elevationaldirection, and said second group of subelements of each of said secondand third groups of elements is located about a third longitudinal axisof said array which is offset on the opposite side of said centrallongitudinal axis in the elevational direction from that of said secondlongitudinal axis.
 16. The array of ultrasonic transducer elements ofclaim 15, wherein said second and third longitudinal axes aresymmetrically offset from said central longitudinal axis.
 17. The arrayof ultrasonic transducer elements of claim 11, wherein the subelementsof each of said groups of subelements are located along a respectiveline in the plane of said array which is angled longitudinally outwardfrom a longitudinal side of said first group of elements and angled atan increasing distance from said central longitudinal axis as it extendsoutward from said first group of elements.
 18. The array of ultrasonictransducer elements of claim 11, wherein said elements and subelementsare separately actuatable from each other in the longitudinal directionand in the elevational direction for steering and focusing of said beamin the longitudinal direction of said plane and for spatiallycompounding the transmitted acoustic energy.
 19. The array of ultrasonictransducer elements of claim 10, wherein said first group of elementsexhibits a higher frequency response than said second and third groupsof elements, whereby signals of differing frequency content are receivedby different groups of elements in the far field.
 20. The array ofultrasonic transducer elements of claim 19, wherein said first group ofelements exhibits a lesser thickness in the transmission dimension thansaid second and third groups of elements.
 21. An array of ultrasonictransducer elements arrayed in the longitudinal direction of said arraywhich transmits acoustic beams in a plane extending from a centrallongitudinal axis of said array, comprising:a first group of elementslocated about the longitudinal center of said array and operable for thetransmission of acoustic energy in the near and far fields; and secondand third groups of elements located on opposite sides of said firstgroup of elements in the longitudinal direction, with elements of saidsecond and third groups extending a greater distance in the elevationaldirection from said central longitudinal axis than said first group ofelements, wherein said second and third groups of elements are operablein concert with said first group of elements to provide elevationalfocusing in the far field, wherein said first group of subelements ofeach of said second and third groups of elements is located about asecond longitudinal axis of said array which is offset on one side ofsaid central longitudinal axis in the elevational direction, and saidsecond group of subelements of each of said second and third groups ofelements is located about a third longitudinal axis of said array whichis offset on the opposite side of said central longitudinal axis in theelevational direction from that of said second longitudinal axis,further comprising fourth and fifth groups of elements located onopposite sides of said second and third groups of elements,respectively, each of said fourth and fifth groups comprising first andsecond groups of subelements which oppose each other in the elevationaldirection, wherein said first group of subelements of each of saidfourth and fifth groups of elements is located about a fourthlongitudinal axis of said array which is offset on one side of saidcentral longitudinal axis in the elevational direction to a greaterdegree than said second longitudinal axis, and said second group ofsubelements of each of said fourth and fifth groups of elements islocated about a fifth longitudinal axis of said array which is offset onthe opposite side of said central longitudinal axis in the elevationaldirection from that of said fourth longitudinal axis to a greater degreethan said third longitudinal axis.
 22. A linear array of ultrasonictransducer elements arrayed in the longitudinal direction of said arraywhich transmits acoustic beams in a plane extending from a centrallongitudinal axis of said array, comprising a plurality of elements eachof which is aligned in parallel in the elevational direction with anadjacent element and exhibiting extension in the elevational directionwith respect to said central longitudinal axis which varies in relationto the displacement of said elements from the longitudinal center of thearray.